Inkjet printers have advantages of low-noise, low-running cost, easiness of color printing, so that they have been widely spread in common households as an output apparatus of digital signal.
In recent years, the inkjet printers have been used for industrial use such as a display, a poster, or a bulletin board, as well as for home use. However, in the case of industrial use, a non-porous recording medium such as a plastic film has been used, because a porous recording medium has a disadvantage in durability such as light resistance, water resistance, or wear resistance. Therefore, an ink for the non-porous recording medium has been developed. For example, a solvent based inkjet ink which contains an organic solvent as a vehicle and a UV curable inkjet ink which contains a polymerizable monomer as a main ingredient have been widely used.
However, the solvent based inkjet ink is not preferable from the viewpoint of environmental loads, because it vaporizes a large amount of the solvent therein into the atmosphere. The UV curable inkjet ink may have skin sensitization depending on a type of the monomer to be used. Additionally, an expensive ultraviolet irradiation device should be integrated into a main body of a printer, which limits a field of application of the ink.
In view of the above described background, recently, an aqueous inkjet recording ink has been developed which can directly printed onto a non-porous base (see PTLs 1 and 2).
However, it has been mentioned that the aqueous ink is in some ways inferior to the solvent based inkjet ink with respect to image quality.
Firstly, the non-porous base such as the plastic film easily repels water which is a main ingredient of the aqueous ink, so that ink droplets ejected from a head do not easily wet and spread over the base. As a result, microvoids are left on a solid image, and high image density is difficult to be achieved.
Secondly, the ink basically does not penetrate into the non-porous base. Therefore, the ink ejected onto the base must rapidly dry. However, water serving as a main solvent of the aqueous ink, and a water-soluble organic solvent serving as an additive deteriorate a drying property of the ink, which tends to cause drying failure. Accordingly, set-off of the ink, so-called blocking, may occur when a printed matter is placed on top of another or is rolled up.
Further, the non-porous base often has high glossiness. Therefore, in order to prevent integrity as a recorded matter from being impaired between a printed portion and a non-printed portion upon printing, an ink which can achieve high glossiness has been demanded. However, in the case of aqueous ink, unlike the solvent based ink in which resin is dissolved into the ink, particles are fused to each other to thereby form a coating film, so that a surface thereof tends to be roughened and glossiness tends to be impaired.
Additionally, with respect to image fastness, satisfactory scratch resistance and ethanol resistance have not been achieved. Therefore, the fact is that an aqueous ink having higher performance is demanded.
Meanwhile, an aqueous inkjet ink containing polycarbonate urethane resin particles has been already known before the filing of the patent application (see PTLs 3 and 4). However, an ink composition for the non-porous base has not been examined. There have been examinations only on conventional pigment inks.